NO341378B1 - Process for purifying a gas stream contaminated with CO2 and one or more hydrocarbons and / or nitrogen oxides by absorption on an aggregated zeolite adsorbent - Google Patents

Description

The area of the present invention is the purification of gas streams by adsorption on zeolite adsorbents. More specifically, the invention relates to a method for cleaning a gas stream which, apart from CO2, contains at least one contaminant selected from the group consisting of hydrocarbons and nitrogen oxides (NxOy) and possibly water.

The production of pure gases is an industrial operation carried out on a large scale and which may include either cryogenic processes or adsorption processes or a combination of both these techniques; adsorption processes are generally based on the gas streams to be purified being passed through a layer of adsorbent(s) by applying the principle of PSA (pressure swing adsorption), that of TSA (temperature swing adsorption) or a combination of both (PTSA or TEPSA (thermally enhanced pressure swing adsorption) ; see, eg, US 5,614,000).

The construction of industrial units to produce ever greater quantities of pure gases, e.g. purified air before the step of separating N2 and O2, or otherwise syngas purified before the separation of CO and H2, requires the use of increasing amounts of adsorbents and increases the energy requirements intended to compensate for pressure loss. By improving the adsorptivity, optimizing the material handling properties of the zeolites used in industrial adsorption devices constitutes in this respect a main economic challenge.

To eliminate carbon dioxide from a gas stream, e.g. for the purification of air, it is very common practice in industrial adsorption units to use a layer of adsorbent based on fajasite-type zeolite which has a Si/Al ratio greater than or equal to 1.2, if exchangeable sites are sodium sites, such a zeolite is also designated zeolite 13X. In some cases, it has been known that zeolite X is a better carbon dioxide adsorbent than silica gel or activated carbon (US 2,882,244). This patent also describes that the selectivity with regard to different adsorbates varies with temperature and pressure.

US 3 885 926 describes that the adsorption of CO2 can be effected on a zeolite X yielded to an extent of more than 90% with barium, the CO2 content of the gas to be purified does not exceed 1000 ppm and the temperature can be between -40°C and 50°C.

EP 294 588 describes the use of zeolite X, preferably yielded to the extent of 70% with strontium, to carry out this purification.

US 5 531 801 and EP 718 024 describe that it is possible to adsorb CO2 very efficiently using an X-type zeolite with a Si/Al ratio of less than 1.15 and preferably equal to or very close to 1, designated LSX (low silica X). The advantage compared to a conventional faujasite (Si/Al > 1.2) lies in the fact that it is no longer necessary to reduce the temperature of the decarbonization step by means of a cold unit, since the efficiency of the zeolite is such that the selectivity for CO2 compared to N2 remains high, even at 50°C.

EP 1 062 022 shows that a very significant increase in efficiency can be achieved with respect to decarbonisation at low CO2 partial pressures (of around 2 mbar) by using LSX zeolites whose degree of sodium exchange (defined as the molar ratio of sodium ions to aluminum ions in the tetrahetic position, the rest is potassium) is at least 98%.

WO 00/01478 describes NaKLSX adsorbents in which the Si/Al ratio is between 0.9 and 1.1, the K<+->ion content is less than 8%, the macroporous volume is greater than 0.4 cm/g, containing small crystals ( 1-4 µm) which can be used for the decarbonisation of gas streams. The document shows an increase in dynamic adsorptivity at room temperature in case of low CO2 partial pressure, and an improvement in kinetics and dynamics of PS A and TS A adsorbent layers due to the use of such molecular sieves. The authors attribute the increase in dynamic performance compared to conventional NaKLSX adsorbent (i.e., where the zeolite is not in the form of small crystals) to the increase in macroporous volume leading to rapid diffusion into the grain of the adsorbent and the abilities of small crystals to reduce resistance against surface diffusion compared to coarser crystals.

EP 1 218 099 proposes the zeolite adsorbent suitable for purifying gas streams contaminated by CO2, based on a mixture of X and LSX zeolites, mainly substituted with sodium and strontium, aggregated with an optional zeolitized binder

EP 1 245 266 recommends the use of zeolites type X or type A with a Si/Al ratio > 1.2 aggregated with a binder that is zeolitized, the authors have observed that zeolitization acts to increase the dynamic adsorptivity for CO2 or N2O more than proportionally with the increase in active material (due to zeolitization) and reduces the mass transfer area by at least 10 to 20%.

To take care of industrial equipment, e.g. cryogenic N2O2 separation units, it is a topic to remove, upstream of these units, apart from carbon dioxide and possibly water vapor, on the one hand the nitrogen oxide (hereafter denoted as NxOy with x varying from 1 to 2 and y varying from 1 to 4) which is in capable of solidifying in certain areas of the cold boxes and preventing their proper operation and/or on the other hand hydrocarbons, present in trace amounts (ppm), which may possibly concentrate at certain points and threaten the safe operation of industrial facilities.

One solution consists in stopping the production of clean gas on a given adsorbent layer when one of the CO2NxOy or hydrocarbon-type contaminants breaks into the production yield. When it e.g. if air is purified on a zeolite 13C, which is conventionally used for decarbonisation, the hydrocarbons and nitrogen oxides naturally break through before CO2, so that the adsorbent is regenerated in good time before CO2 breakthrough.

EP 930 089 describes the elimination of CO2 contained in a gas stream and possibly traces of water vapor and hydrocarbons by adsorption of these pollutants on a zeolite X with a Si/Al ratio of between 1 and 1.5, containing 50 to 99% Ca<2+> cations, at most 25% K + cations and at least 1% of Na 2+ cations. The process is carried out at a temperature between -40°C and 80°C, preferably by TSA.

EP 1 064 978 Al describes the use of a zeolite of type X and LSX (Si/Al = 1 to 1.4) yielded to an extent of at least 30%, preferably at least 75%, with barium and containing cations from group IA or Ila for air purification by adsorption of CO2, but also of C3H8, C2H4 and N2O type pollutants. Such an adsorbent has the disadvantage that it is more expensive than a conventional 13X type of adsorbent, although it turns out to be more effective.

FR 2 832 077 proposes an X-type adsorbent yielded with calcium and with barium which is easier to prepare and less expensive than a high barium yield X as described in EP 1 064 978 and also capable of purifying air by eliminating hydrocarbons such as ethylene, acetylene, butane and propane, and also nitrogen oxides NxOy, while removing CO2 and water vapor contained in the gas stream.

Other solutions have also been proposed to clean air containing several pollutants, including water vapour, carbon dioxide, nitrogen oxides NxOy and/or traces of hydrocarbons consisting of using layers of several layers of adsorbents or composite adsorbents consisting of aggregated mixtures based on different zeolites.

EP 862 938 A1 describes a PS A process for eliminating CO2 water and NxOy, where the adsorbent system is an aluminum oxide/zeolite double layer, the zeolite possibly being of Y, X or A type.

EP 992 264 A1 describes a method for eliminating CO2, water and NxOy and optionally ethylene, where the adsorbent system is of a three layer comprising, in order, an adsorbent which is selective for water vapour, then an adsorbent which is selective for CO2 (preferably 4A , 5A or 13X-based zeolite) and finally third adsorbent selective for NxOy and optionally for ethylene (preferably based on zeolite CaX BaX or binderless (with a low amount of binder) CaX).

EP 1 092 465 A2 describes a TSA process for the elimination of CO2, water NxOy and/or hydrocarbons containing, in order, an adsorbent which is selective for water vapour, then an adsorbent which is selective for CO2 (preferably based on NaLSX zeolite) and finally a third adsorbent selective for NxOy and/or for hydrocarbons (preferably based on CaX or CaLSK zeolite).

EP 904 823 A2 and US 5 906 675 describe a method for eliminating CO2 which comprises a first layer of adsorbent capable of adsorbing water and at least 75% of CO2 (preferably aluminum oxide) operated in PSA mode and a second layer of adsorbent which is capable of adsorbing CO2 (preferably NaLSX) operated in TSA mode.

EP 862 936 A2 describes a PSA process for eliminating CO2, water and NxOy, where the adsorbent is aluminum oxide/zeolite mixture, it being possible for the zeolite to be of Y, X, LSX or A type.

EP 1 101 521 Al describes a TSA process for the elimination of CO2, water and NxOy and/or hydrocarbons, whose adsorbent layer consists of a mixture of an adsorbent for water and CO2 (e.g. based on a NaX or NalSX zeolite) and a adsorbent for NxOy and/or hydrocarbons (e.g. based on a CaX or CaLSX zeolite).

WO 01/24923 A describes a method for purifying a gas stream containing CO2. The gas stream is brought into contact with a zeolite absorbent comprising a mixture of 5-95% by weight of at least one zeolite X and 95-5% by weight of at least one zeolite LSX in which at least 80% of the sum of the exchangeable cation sites of all the zeolites in the mixture can be concerned with sodium cations.

EP 1 468 731 A relates to a method for purifying a gas stream containing carbon dioxide, nitrous oxide and/or hydrocarbons. The gas stream is purified by contacting it with a zeolite comprising LSX with up to 100% mixed with 13X. The zeolite can be exchanged with sodium, and the absorbent contains a binder.

The present invention proposes a method for purifying a gas stream which, as pollutants, contains at least carbon dioxide, hydrocarbons and/or NxOy and possibly water, by adsorption on an aggregated zeolitic adsorbent, this method of purification proves to be particularly effective with regard to elimination of all the above-mentioned pollutants. The dynamic CO2 adsorption activity is high without leading to premature breakthrough of hydrocarbons and/or NxOy, which does not require the use of multiple layer layers or systems of composite layers (several adsorbents for the same layer). Since, in addition, the adsorbent used does not require exchange with barium or calcium cations, like the technical solutions proposed in EP 995 477 or EP 1 064 987, the method for purification according to the present invention is particularly cheap.

The method for purifying a gas stream containing, in addition to carbon dioxide, at least one contaminant comprising hydrocarbons and nitrogen oxides, possibly water, according to the invention consists in: A- bringing the gas stream to be purified into contact with at least one adsorption agent

in aggregated form

comprising either at least 70%, preferably at least 80% and advantageously at least 85% by weight, of zeolite LSX, whose crystals have a number average diameter measured by SEM and counting, of less than or equal to 4 µm, at least 90%, preferably at least 95% and advantageously 98% of the exchangeable sites on this are taken up with sodium ions, the rest of the cations possibly being e.g. potassium cations, up to 5% by weight of a binder which is inert with respect to adsorption, and optionally up to 25%, preferably up to 20% and advantageously up to 15% of one or more other zeolites, such as zeolite X, zeolite A, etc. (type A aggregate adsorbent);

or comprising at least 70%, preferably at least 80% and advantageously at least 85% by weight of a mixture of at least 20%, and preferably at least 30% of zeolite X and of at most 80%, preferably at least 70% by weight of zeolite LSX, if crystals have a number average diameter, measured by SEM and counting, of less than or equal to 4 µm, where

at least 90%, preferably at least 95% and advantageously at least 98% of the exchangeable sites of the aforementioned zeolites X and LSX are occupied by sodium ions, the rest of the cations possibly being e.g. potassium ions, up to 5% by weight of a binder which is inert to the adsorption and optionally up to 25%, preferably up to 20% and advantageously up to 15% of one or more other zeolites, such as zeolite A, etc. (type B aggregate);

B- adsorption of at least some of the carbon dioxide, and at least some of the hydrocarbons

and/or NxOy on said adsorbent; C- desorption of the pollution adsorbed on said adsorbent, and

D- regeneration of the adsorbent.

The term "zeolite LSX" shall be understood to mean an X-type zeolite (faujasite) having a Si/Al ratio of 1.00 ± 0.02, the crystals of which have a number average diameter, measured by SEM and counting, of less than or equal to 4 µm, advantageously less than or equal to 3 µm and even more advantageously less than or equal to 2 µm.

The term "zeolite X" is understood to mean an X-type zeolite (faujasite) having a Si/Al ratio of 1.25 ± 0.05, whose crystals preferably have a number average diameter of less than or equal to 4 µm, preferably less than or equal to 3 µm and advantageously less than or equal to 2 µm.

The aggregated zeolites that can be used according to the invention can in particular be in the form of spheres, extrudates or threads, which generally have an average particle size between 0.2 and 5 mm.

A method for producing type A or B zeolitic aggregates that can be used in connection with the invention comprises the following steps;

(1) aggregation of the precursor product, i.e. LSX-type zeolite or a mixture of the zeolite LSX and X initially in a powdered state, with a binder which can convert this into a zeolite; (2) drying at low temperature (around 80-100°C) and calcining at a temperature between 300 and 700°C, preferably between 400 and 600°C, of the product obtained by (i); (3) zeolitization of the binder and simultaneous or subsequent, partial or complete introduction into the zeolite structure of NA<+> ions instead of the already present exchangeable cations (especially K<+>) so that a final sodium yield content of greater than or equal to 98%;

(4) washing the product obtained by (3); and

(5) drying and activating at a temperature between 300 and 700°C, preferably between 400 and 600°C, preferably in a stream of dry, decarbonated air, advantageously in a continuously traversed bed, of the product obtained by (4).

The binder that can be converted to a zeolite can be selected from clays that can be zeolitized, such as kaolin, metakaolin and halloysite, by itself or as a mixture.

For 100 parts by weight of the final aggregate according to the invention, 5 to 25, preferably 5 to 20 parts by weight of binder which can be converted into a zeolite per 95 to 75 parts by weight of zeolite powder are generally used.

The zeolitization step (3) consists in converting all or part of the binder which can be converted into a zeolite with which LSX-type powders are premixed by alkaline softening, preferably according to the teachings of WO 99/05063.

A variant of step (1) consists in the conventional mixing of an LSX-type powder with water and the binder that can be converted into a zeolite and then in spraying this mixture on zeolite aggregates that have already been mixed, as an effective aggregation seed. During this spraying operation, the aggregates can be continuously rotated around themselves by using a "snowball" type technique, e.g. in a reactor equipped with a rotating shaft. The aggregates thus obtained are then in the form of beads.

A particularly preferred method for producing B-type zeolitic aggregates which can be used within the scope of the invention, which is described in EP 1 218 099 and US 6 616 732, consists, before step (1), in carrying out a sodium exchange on X and /or the LS V powder before mixing, or just after mixing.

It would not be outside the scope of the invention to use A- or B-type zeolitic aggregates as defined above which would have been produced by using another production process, e.g. by direct aggregation of the zeolite X and/or LSX mixture with the inert binder without a zeolitization step; in this step, the aggregates would be produced with no more than 5 weight or binder per 95 weight of zeolite LSX or zeolite X/LSX mixture.

Similarly, it would not be outside the scope of the invention to use A- or B-type zeolitic aggregates as defined above which would be produced exclusively from an aggregate binder which can be converted into a zeolite and which is then zeolitized as described e.g. in US 3,119,669, US 4,603,040 and JP 05,163,015.

The gas stream purification method according to the invention can be carried out by passing the gas stream over one or more layers of A-type and/or B-type aggregate zeolitic adsorbent which are connected in parallel or which are capable of cyclically connecting an adsorption step with a desorption step (intended for regeneration of the adsorbent), on an industrial scale, this is preferably carried out using a pressure swing adsorption (TSA), temperature swing adsorption (TSA) or beneficial pressure-temperature swing adsorption (PTA or TEPSA) process. The PSA and PTSA or TEPSA processes involve the application of pressure cycles. In a first phase, the adsorbent layer separates the pollutant by adsorbing this component, in a second phase the adsorbent is regenerated by reducing the pressure and/or by raising the temperature. At each new cycle, it is essential that the desorption of the pollutant is as complete as possible and as efficient as possible, in order to be able to return to a regenerated adsorbent state that is identical or nearly identical at each new cycle.

The cleaning method according to the invention is particularly suitable for cleaning air and for cleaning a syngas.

For continuous purification of the gas stream, such as air or a syngas, a number of adsorbent layers are generally placed in parallel, these being alternately subjected to a cycle consisting of adsorption with compression and desorption with decompression. In the PSA and PTSA processes, the treatment comprises the cycle in which each layer is subjected to the following steps: a/ the contaminated gas stream is made to pass through an adsorption area comprising the adsorption layer, the adsorption layer separating the contaminant or contaminants by adsorption; bl desorption of the adsorbed pollutants by establishing a pressure gradient over progressively reducing the pressure in said adsorption area in order to extract the pollutants via the inlet in the adsorption area; and c/ raising the pressure in said adsorption area by introducing a flow of clean gas via the outlet from the adsorption area.

Accordingly, each bed is subjected to a treatment cycle comprising a clean (purified) gas production phase, a second decompression phase and a third recompression phase. The adsorption area can, next to an adsorbent layer described above, include one or more other adsorbent layers capable of adsorbing the unwanted pollutants or contaminations (multilayer system and/or mixture of several adsorbents in the same layer). For e.g. to remove the water contained in air, it is possible to combine a desiccant to adsorb water, such as aluminum oxide or a silica gel, with the adsorbent according to the present invention, either in the same layer or by superposition of two layers.

To optimize the PSA and PTSA processes, the phases with decompression and/or compression of the different adsorbent layers are preferably synchronized. It turns out to be particularly advantageous to introduce steps to equalize the pressures between two adsorbent layers, one being in the decompression phase, the other in the recompression phase. During implementation of the cleaning method according to the invention, using PSA or PTSA to clean air, the adsorption pressures are generally between 0.02 and 2 MPa, and preferably between 0.11 and 1 MPa, while the desorption pressures are generally between 0.002 and 0.5 MPa and preferably between 0.01 and 0.2 MPa. As in the case of the prior art air cleaning processes, the temperatures in the adsorption range are generally between + and 80°C, advantageously between 20 and 60°C, and the desorption temperatures may generally be between 80 and 300°C, preferably between 150 and 250°C.

The amount of CO2 present in the contaminated air gas stream is generally between 400 ppm and 0.7% by weight.

The amount of hydrocarbons present in the contaminated air gas stream generally does not exceed a few hundred ppm.

The amount of NxOy present in the contaminated air gas stream generally does not exceed a few ppm.

The generic term "syngas" is used for gases consisting mainly of hydrogen and CO (approx. 25% CO by volume) which are used as reaction products in certain basic chemical syntheses (methanol, acetic acid, phosgene, acrylic compounds, etc.). These syngases are generally obtained by a partial steam oxidation or CO2 reforming reaction carried out on a hydrocarbon feedstock (ranging from natural gas to heavy hydrocarbons) which gives a mixture comprising

H2+ CO + CO2+ H2O other pollutants, as the respective proportions of H2, CO, CO2 and H20 depend on the synthesis conditions.

The term "syngas" is also understood within the present invention and means H2N2 mixtures used especially for synthesizing ammonia. These mixtures are generally produced by partial oxidation of air or reforming of a hydrocarbon feedstock. This step can be supplemented with the so-called "CO shift" reaction, namely CO + H2O —» CO2+ H2, which converts CO into CO2 and consequently produces more hydrogen.

It is often necessary to purify syngas, e.g. when it is desired to separate either CO and H2, or N2 and H2, this being achieved either cryogenically or by scrubbing with liquid methane; it is absolutely necessary to eliminate all impurities that can crystallize and therefore block the exchangers used in the cryogenic process.

If the amount of CO2 contained in the flow of syngas to be cleaned is after a few thousand ppm, it is generally practice to first scrub with amines (of the MEA or MDEA type) to remove most of the CO2. The syngas is then sent into a column of adsorbent(s) to remove the remaining traces of CO2 (a few tenths of a ppm) that have not been removed by the amine scrubbing and possibly the other pollutant or pollutants in the syngas, e.g. water which is often present at the same time as CO2 (as a result of scrubbing with amines, the gas being saturated with water).

As a general rule, the adsorption area is maintained at a pressure between 0.5 and 7 MPa, while the gas mixture to be purified is brought into contact with the adsorbent(s) described above. Although a higher pressure would not destroy the cleaning operation, generally pressures above 7 MPa are avoided for the purpose of saving energy and because of the high cost of pressure-resistant facilities. Pressures below 0.5 MPa are not normally used for the industrial production of syngas for practical reasons, since the processes involving upstream of the purification according to the invention, which correspond to syngas production reactions, are carried out at pressures which are generally approx. 2-3 MPa. Preferably, the pressure obtained in the adsorption region is kept at a value of less than or equal to 5 MPa, and advantageously less than or equal to 3 MPa. Correspondingly, the adsorption range is preferably kept as greater than or equal to 0.5 MPa, or advantageously higher than or equal to 2 MPa.

The temperatures of the gas stream entering the adsorption area are not a determining factor and are generally kept constant throughout the adsorption phase. Usually this temperature is between 0 and 80°C, preferably between 20 and 60°C. The desorption temperature can be between 80 and 300°C.

The amount of CO2 present in the contaminated syngas stream is generally between 50 ppm and 0.7% by weight.

The amount of hydrocarbons present in the contaminated syngas stream generally does not exceed a few hundred ppm.

The amount of NxOy present in the contaminated syngas stream generally does not exceed a few ppm.

The examples below, which are provided by way of non-limiting illustration, demonstrate the advantage of such a purification method.

The dynamic adsorption cycle of the zeolite adsorbent was assessed by a conventional advancing front chromatography test which consists in measuring, at the outlet of a column filled with aggregated adsorbents, the concentration response to a concentration step injected at the inlet of the column. Figure 1 shows the principle of this step: a column of the adsorbent is swept by a carrier gas. After time t = 0, the gas to be tested, hereinafter referred to as "gas A" is injected at a concentration co at the inlet of the column. The concentration c of gas A at the outlet of the column is determined by an analyzing device and recorded as a function of time. The curve c as a function of t, denoted the "breakthrough curve", constitutes an evaluation of the dynamic adsorptivity and the kinetic adsorption factors of the zeolite adsorbent with regard to gas A. Two characteristic quantities, which for a person skilled in the art quantify the performance of the zeolitic adsorbent, determine from the breakthrough curve: first, the "breakthrough time" which corresponds to a concentration c at the outlet

equal to a small percentage of the inlet concentration co, e.g. 5 ppm, and

second, the "mass transfer zone", namely the estimated average thickness of adsorbent that is neither deficient in adsorbed gas A nor saturated with gas A (intermediate region).

The breakthrough time is characteristic of the dynamic adsorptivity of the zeolitic adsorbent.

The mass transfer zone (MTZ) is an indicator of the diffusion properties and adsorption kinetics of the adsorbent and is calculated as follows:

where t; is the time after which the concentration of gas A at the outlet is equal to a low percentage, e.g. 1 to 5%, of the inlet concentration Co, tf is the time after which it is equal to Co/2, tf is the time after which the concentration of gas A at the outlet is equal to a high percentage, e.g. 95 to 99%, of the inlet concentration co and H is the filling height of zeolitic adsorbent in the column.

A minimal transfer zone, i.e. a steep front, means that a given adsorbent layer can be operated with a maximum thickness of saturated adsorbent relative to the thickness of the layer, i.e. with maximum adsorptivity.

The steepness of the front opens the way to reduce the layer thicknesses in TPSA plants and to their operation with shorter cycles while also being able to produce clean gases.

The dynamic adsorption test is carried out in a column with a diameter of 29.7 mm with a filling height H of 42 cm. The carrier gas is nitrogen and this is injected at the bottom of the column with a volume flow rate of 2360 Sl/hour. Gas A is carbon dioxide and is mixed with a carrier gas at time t = 0 to be present in the stream at the inlet of the column with a concentration of 450 ppmv. The column is maintained at a pressure of 6 bar with a temperature of 25°C. The breakthrough time corresponds to a CO2 concentration of 5 ppmv at the outlet of the column. The mass transfer zone MTZ is calculated by applying the formula given above, with the times t; and tf taken for outlet concentrations equal to 5% and 95% of the inlet concentration, respectively.

In all the examples, unless otherwise stated, the proportions are expressed by weight.

The following Examples 1 to 3 including Tables 1 to 3 are for illustrative purposes. Example 4 with table 4 and table 4bis demonstrates the invention using absorbents according to the invention and comparable materials.

Example 1 (adsorption of CO2)

The mass transfer zones for CO2 and the breakthrough times for several zeolitic adsorbents prepared by aggregating 85 parts by weight of zeolite X powder, LSX powder and an X/LSX powder mixture with 15 parts by weight and a binder that can be converted into a zeolite, of the kaolin type, regardless of whether it is subsequently zeolitized or not , were compared.

The zeolitization of the binder was carried out according to the method indicated in steps (1) to (5) which are discussed in detail above (the activation temperature in step (5) being approximately 580°C). After zeolitization, the aggregates with zeolitized binder contained a maximum of 5% by weight of inert material measured by conventional methods, such as the Dubinin volume and the toluene adsorption index (toluene adsorptivity at a relative pressure of 0.5 at 25°C).

Table 1 combines the results from 10 tested zeolitic adsorbents, which were in the form of beads with an average particle size between 1.6 and 2.5 plus and a volume average diameter of 2 mm.

Table 1 clearly illustrates and confirms the advantage of using the aggregates with zeolitized binder compared to conventional 13X zeolite aggregates (with 15% binder) (+ 110 to + 102% increase in breakthrough time) or 13X aggregates with zeolitized binder (+ 3% to 96% increase) in breakthrough time).

It also appears that by replacing a conventional aggregated 13X sieve with 15% binder at the aggregates with zeolitized binder, the dynamic adsorptivity is allowed to be increased by 79 to 184%.

The above test is repeated on 10 zeolitic adsorbents that have the same compositions, but whose activation temperature is approx. 450°C. The results are combined in table 1 bis.

Example 2 (adsorption of a NiO

Nitrogen-kissed adsorption isotherms were determined at 25°C by a conventional volumetric system suitable for low pressure. Table 2 shows the relative Henry constants for these isotherms.

As in the case of CO2 adsorption, it appears that there is an advantage in replacing a conventional 13X sieve with the aggregates with zeolitized binder.

Example 3 (adsorption of ethylene)

Ethylene adsorption isotherms were similarly determined at 25°C. The table shows the relative amounts adsorbed at the different sieves under a pressure of 3 mbar.

It appears that most of the aggregated zeolites with a zeolitized binder show the highest ethylene adsorptivity.

Example 4 (effect on zeolite crystal size (adsorption of CO2) The particle size of the zeolite LSX and X powders used was determined using a commercial apparatus from Malvern, based on light scattering. The number average diameters calculated by the working software are indicated in the second column of table 4 and illustrate the effect of the size of the crystals used for the aggregation Table 4 repeats and supplements the results in Table 1, in that the mean diameter of the zeolite powders used for the production of the aggregates is indicated and is supplemented by results obtained by using crystals of smaller particle size, in which the number means of the zeolite LSX crystals are 6.5 µm and the zeolite X crystals are 2.3 µm (also activated at approx. 580° C). Table 4 clearly shows the consequence of reducing the crystal size of the aggregates according to the invention.

Aggregates made up of smaller crystals are marked with advantageous equivalents made up of coarser crystals, both with regard to relative breakthrough time and mass transfer zones, which is unexpected and makes the small crystal aggregates according to the invention particularly attractive for decarbonisation.

Claims (9)

Pa ten tkra v 1. Process for cleaning a gas stream containing, in addition to carbon dioxide, at least one contaminant comprising hydrocarbons and nitrogen oxides and possibly water, characterized in that it consists of: A – to bring the gas stream to be purified into contact with at least one absorbent in aggregate form either at least comprising 70%, preferably at least 80% and advantageously at least 85% by weight of zeolite LSX, whose crystals have a number average diameter measured by SEM and counting, of less than or equal to 4 μm, of which at least 90%, preferably at least 95% and advantageously at least 98% of the exchangeable sets are taken up by sodium ions, in that the rest of the cations are possibly e.g. potassium cations and up to 5% by weight of a binder which is inert to adsorption, and optionally up to 25%, preferably up to 20% and advantageously up to 15% of one or more other zeolites, such as zeolite X, zeolite A, etc. (type A unit); or comprising at least 70%, preferably at least 80% and preferably at least 85% by weight of a mixture of which at least 20% is advantageous, at least 30% zeolite X and at most 80%, preferably at least 70% by weight of zeolite LSX, whose crystals has a number average diameter measured by SEM and counting, of less than or equal to 4 μm, of which at least 90%, preferably at least 95% is advantageous at least 98% of the exchangeable sets of said zeolite X and LSX are occupied by sodium ions, the rest of the cations possibly your e.g. potassium cations, and up to 5% by weight of a binder which is inert to adsorption, and optionally up to 25%, preferably up to 20% is advantageous and up to 15% of one or more other zeolites, such as zeolite A, etc. (type B unit); B- adsorption of at least some of the carbon dioxide, and at least some of the hydrocarbons and/or NxOy on said adsorbent; C- desorption of pollution adsorbed on said adsorption agent and D- regeneration of the adsorbent. 2. Method according to claim 1, characterized in that a PSA, TSA, TPSA or TEPSA-type process is carried out. 3. Method according to claim 1 or 2, c a r a c t e r i s e r in that the adsorbent in the form of a type A aggregate is prepared by using a manufacturing process comprising the following steps: (1) aggregation of the precursor product, ie. The LSX-type zeolite initially in a powdered state, with a binder that can be converted to zeolite; (2) drying at low temperature (around 80-100<o>C) and calcination at a temperature between 300 and 700<o>C, preferably between 400 and 600<o>C, of the product obtained by (1); (3) zeolitization of the binder and simultaneous or subsequent, partial or complete introduction into the zeolite structure of NA<+> ions instead of the already present exchangeable cations (especially K<+>) so that a final sodium exchange content of greater than or equal to 90%, preferably greater than or equal to 95% and advantageously greater than or equal to 98%; (4) washing the product obtained by (3); and (5) drying and activation at a temperature between 300 and 700<o>C, preferably between 400 and 600<o>C, preferably in a stream of dry decarbonated air, advantageously in a continuous traversed layer, of the product obtained by (4 ). 4. Method according to claim 1 or 2, c a r a c t e r i s e r in that the adsorbent in type B aggregate form is produced by using a manufacturing process that includes the following steps: (1) aggregation of the precursor product, ie. the mixture of LSX-type and X-type zeolites initially in a powdered state, with a binder which can be converted into a zeolite; (2) drying at low temperature (around 80-100<o>C) and calcining at a temperature between 300 and 700<o>C, preferably between 400 and 600<o>C, of the product obtained in (1); (3) zeolitization of the binder and simultaneous or subsequent, partial or complete introduction into the zeolite structure of Na<+> ions instead of the already present exchangeable cations (especially K<+>) in order to have a final sodium exchange content of greater than or equal to 90%, preferably greater than or equal to 95% and advantageously greater than or equal to 98%; (4) washing the product obtained by 3; and (5) drying and activation at a temperature between 300 and 700<o>C, preferably between 400 and 600<o>C, preferably in a stream of dry decarbonated air, advantageously in a continuous traversed layer, of the product obtained by (4 ), and preferably before step (1) the X and/or LSX powder has undergone a sodium exchange, either before they are mixed or immediately after they are mixed. 5. Method according to any one of claims 1 to 4, characterized in that the adsorbent in type A or B aggregate form is produced by using a manufacturing process comprising the direct aggregation of 95 parts by weight of zeolite LSX or an X/LSX mixture with a maximum of 5 weight or binder, followed by sodium exchange and activation. 6. Method according to any one of claims 1 to 4, characterized in that the adsorbent in type A or B aggregate form is prepared by aggregating a binder which can be converted into a zeolite, followed by zeolitization of the binder essentially into LSX zeolite and/ or LSX and X zeolites and activation of the adsorption agent. 7. Method according to any one of claims 1 to 6, characterized in that the gas stream to be purified is based on air. 8. Method according to any one of claims 1 to 6, characterized in that the gas stream to be purified is based on syngas. 9. Method according to any one of claims 1 to 8, characterized in that the adsorbent layer is a multilayer system that is made up of several different superimposed adsorbent layers and/or comprises at least one layer that comprises a mixture of several absorption agents.